The present invention relates to a sealed rechargeable battery having a zinc electrode and, more particularly, to a configuration of a battery having one zinc electrode and another electrode of, for example, nickel or silver, which is rechargeable and which is sealed.
Zinc-nickel and zinc-silver batteries possess certain desirable qualities. Chief among these is their high energy density, or high energy output per unit weight or volume, which is roughly three to five times higher than that of a lead-acid or nickel-cadmium battery.
Since zinc-nickel and zinc-silver batteries are normally more expensive than the comparable lead-acid battery, it is highly desirable to use these zinc batteries as a secondary battery with alternating discharging and charging cycles.
However, for a number of reasons nickel-zinc and silver-zinc batteries perform inadequately as a secondary cell, and suffer from a limited cycle life. One of the reasons for this is related to the fact that presently known nickel-zinc and silver-zinc batteries are unsealed, that is, open to the atmosphere. The batteries are unsealed in order to better manage the production and consumption of gases during cell charging. The problem is made more severe by the presence of a cellophane separator between the electrodes whose small pore structure size makes it difficult if not impossible for the oxygen cycle to take place.
Contact with the atmosphere can lead to the carbonization of the electrolyte through reaction of the potassium hydroxide electrolyte with carbon dioxide in the air according to the reaction: EQU CO.sub.2 +2 KOH.fwdarw.K.sub.2 CO.sub.3 +H.sub.2 O (1)
Operating a conventional zinc battery in a cycling mode has a number of other highly unpleasant consequences. For example, it is relatively difficult to accurately determine when the cell has been fully charged. This uncertainty can lead to undercharging which, in turn, increases cycle frequency and operational inconvenience. Alternatively, the uncertainty as to the end point of charging can lead to overcharging.
Overcharging can have a number of serious detrimental consequences. First, the release of oxygen at the nickel or silver electrode and of hydrogen at the zinc electrode can bring about the deterioration of the electrolyte during overcharging when oxygen is evolved at the positive electrode and hydrogen is evolved at the negative electrode (See reactions (5) and (6) below). The net effect is to eliminate water (see reaction (7) below) which tends to concentrate the electrolyte.
Second, overcharging increases the possibility of the irreversible formation of dendrites, which consist of low zinc ion regions, which tend to short-circuit the cell and destroy its effectiveness.
For these and other reasons, there is thus a widely recognized need for a sealed rechargeable nickel-zinc or nickel-silver cell which can be charged optimally without overcharging or undercharging and which will not deteriorate rapidly during use.
It would be desirable to have a rechargeable zinc battery which can be sealed by the manufacturer so as to eliminate all contact with the atmosphere and thereby reduce the attendant deleterious effects of such contact. There is also a clear need for, and it would be highly advantageous to have, a sealed rechargeable zinc battery which could be fully charged without fear of overcharging and the adverse- consequences of such overcharging.